1. Field of the Invention
The present invention relates to a semiconductor device, and more particularly relates to a semiconductor device that enables output drivers to function as termination resistors.
2. Description of Related Art
Conventional memory modules have termination resistors including discrete components mounted on a module substrate to suppress reflection of signals. However, a semiconductor device having an ODT (On Die Termination) function that causes output drivers to function as termination resistors is recently used to reduce the number of components or dynamically change values of termination resistance. For example, in a semiconductor device described in Japanese Patent Application Laid-open No. 2008-210307, an output driver connected to a DQ pin and an output driver connected to a DQS pin function as termination resistors when a control signal ODTON is activated.
However, during a write operation of a semiconductor device, a controller device does not always output a data signal DQ and a data strobe signal DQS to the semiconductor device at the same timing and, for example, an output timing of the data signal DQ is sometimes delayed by a predetermined time from an output timing of the data strobe signal DQS. In this case, if output impedances of output drivers are changed at the same time as in the Japanese Patent Application Laid-open No. 2008-210307, resistance values of the termination resistors may be changed during a burst input of the data signal DQ, which prevents a correct write operation. To solve this problem, it suffices to control the resistance values of the termination resistors to be kept until the burst input of the data signal DQ ends. In this case, however, start of the next write operation or read operation has to be delayed and access efficiency is decreased.